The development of radiofrequency telecommunications over the last ten or so years has been reflected in congestion of the authorized frequency bands. To exploit the available frequency bands, the systems have to include a band filtering means with a narrow transition band. Only the resonator filters in SAW (surface wave) or BAW (bulk wave) technology, using the piezoelectric properties of the materials, can satisfy these specifications with low losses and a reduced congestion. These days, the piezoelectric layers used for these filters are produced by deposition which makes it possible to design bulk acoustic wave filters (BAW filters) or from bulk substrates that make it possible to design surface acoustic wave filters (SAW filters).
The specifications demanded by the professionals in telecommunications equipment are increasingly difficult in terms of out-band rejection and transition stiffness, without in any way being relaxed with regard to the other parameters (insertion losses, in-band variations, etc.). The only way to give back a little design latitude consists in significantly reducing the frequency variations due to temperature.
It will be recalled, by way of indication, that the variation of the resonance frequency of a resonator on quartz is determined by the following formula:f(T)=f0[1+CTF1(T−T0)+CTF2(T−T0)2+ . . . ]with f0 being the frequency at T0, T0 being the reference temperature (25° C. by convention), CTF1 being the first order coefficient expressed in ppm/° C. and CTF2 being the second order coefficient expressed in ppb/° C.2.
For CTF1, by changing, for example, from 35 ppm/° C. to 20 ppm/° C. or less, it is possible to improve the performance levels significantly and thus the competitiveness of the products.
FIG. 1a schematically represents an SAW filter structure: on a bulk piezoelectric substrate Spiezo,s, surface metallizations, which can typically be of interdigital electrode comb type Ms1 and Ms2, ensure the excitation of the piezoelectric material allowing for the propagation of surface acoustic waves.
The surface wave filters are still currently the standard solution for synthesizing and producing RF filters in the 50 MHz-3 GHz range given their robustness, their technological ease of implementation and the multitude of accessible filter structures, offering the designer a true design strategy based on the specifications imposed.
For example, and although affected by thermal drifts, the filters on tantalate and lithium niobate make it possible to produce relative bandwidths greater than those of the bulk wave filters by virtue of their high coupling coefficients (more than 10%) and their quality factor Q. Thus, it is possible to obtain products Q×fmax of the surface waves close to 1013 whereas those obtained with the bulk waves on thin films remain around 4×1012.
FIG. 1b schematically represents a bulk wave filter structure: a piezoelectric substrate Spiezo, v is inserted between two metallizations M1 and M2 allowing for the propagation of bulk waves.
The bulk wave filters were proposed approximately thirty years ago, with frequencies of from a few MHz to a few tens of MHz, mainly using impedance elements or lateral coupling structures on quartz for narrow band applications, but their implementation for radio frequencies dates back only ten or so years, following the pioneering works of Lakin (K. M. Lakin and J. S. Wang, UHF composite bulk wave resonators, 1980 IEEE Ultrasonics Symposium Proceedings, pages 834-837) concerning the use of piezoelectric layers deposited by cathodic sputtering for such purposes.
The company Agilent was the first to develop an RF filter of FBAR (film bulk acoustic resonator) filter type based on impedance elements exploiting thin films of aluminium nitride (AIN), a deposited polycrystalline material.
The BAW resonators exploit the thickness resonance of a thin piezoelectric layer which is acoustically insulated from the substrate either by a membrane (FBAR technology used by the company AVAGO Technologies), or by a Bragg array (SMR technology used by the company Infineon).
The material most widely used in BAW technology is currently aluminium nitride (AIN), which offers the advantage of exhibiting piezoelectric coupling coefficients of the order of 6.5%, and also of exhibiting low acoustic and dielectric losses, which allows for the synthesis of filters exhibiting passbands compatible with the specifications demanded by most of the telecommunication standards located between 2 and 4 GHz.
However, a number of problems continue to arise, faced with the extremely restrictive specifications presented by a few frequency bands, such as the Digital Cellular Service (“DCS”) standard.
The piezoelectric coupling coefficients permitted by AIN do not allow relative passbands greater than 3%. Such bandwidths already require the use of electrodes having a very strong acoustic impedance (made of molybdenum or of tungsten), so as to contain the elastic energy in the piezoelectric layer, and thicknesses that are carefully determined so as to maximize their influence on the piezoelectric coupling coefficient of the resonators as described in the paper by R. Aigner, “Bringing BAW technology into volume production: the Ten Commandments and the seven deadly sins”, Proceedings of the third international symposium on acoustic wave devices for future mobile communication systems (2007) and in the publication by J. Kaitila, “Review of wave propagation in BAW thin film devices: progress and prospects”, Proceedings of the 2007 IEEE Ultrasonics Symposium.
There are currently no credible solutions for extending this band in relation to constant losses. Numerous research efforts are currently being conducted to find other materials exhibiting higher piezoelectric coupling coefficients, but it has to be stated that it is difficult to uncover other materials offering low acoustic losses and that can be deposited reproducibly and uniformly as described in the paper by P. Muralt et al., “Is there a better material for thin film BAW applications than AIN”, Proceedings of the 2005 IEEE Ultrasonics Symposium.
Conversely, monocrystalline materials such as lithium niobate or lithium tantalate offer very high electromechanical coupling coefficients, making it possible to produce filters exhibiting relative bandwidths of the order of 50% but they remain complex to implement.
Moreover, standards such as DCS require both a wide passband and a strong rejection of the adjacent standards. To simultaneously address all these constraints entails using resonators that have very strong quality coefficients.
Because of this, the limits imposed by the materials themselves, more than by the structure, are beginning to be profiled, and it is highly probable that polycrystalline materials can no longer ultimately meet the rise in quality coefficients, especially given the rise in frequency of the standards toward 10 GHz. With intrinsic quality coefficients of the order of ten or so thousand at frequencies greater than 1 GHz as described in the paper by D. Gachon et al., “Filters using high overtone bulk acoustic resonators on thinned single-crystal piezoelectric layer”, submitted at the 2008 European Frequency and Time Forum, the monocrystalline materials standout here also as an interesting solution.
The BAW resonators based on aluminium nitride AIN use a longitudinal vibration mode capable of generating an acoustic radiation in the air, which is a source of additional acoustic losses. Furthermore, the volume changes associated with the compression mode result in increased acoustic losses inside the material, notably described in H. L. Salvo et al., “Shear mode transducers for high Q bulk microwave resonators”, Proceedings of the 41st Annual Frequency Control Symposium (1987).
One means of reducing the two sources of losses is to exploit shear waves, which requires the use of materials other than AIN, or the crystalline orientation of deposited AIN layers to be modified.
Studies are being focused on the deposition of AIN layers with a crystallographic axis c in the plane of the substrate, or with an angle of 35° relative to the normal, but the quality of the material obtained has proven less good than in the case of a vertical axis c as demonstrated by the authors J. S. Wang et al., “Sputtered c-axis piezoelectric films and shear wave resonators”, Proceedings of the 37th Annual Frequency Control Symposium (1983).
It emerges from this prior art that, in general, each type of filter offers advantages and drawbacks. For example, the SAW filters are known for having strong coupling coefficients allowing for a higher rejection rate. The BAW filters are used more for their low insertion losses and their more effective temperature compensation than on SAWs.
Thus, it may be advantageous to produce a filter combining SAW filter and BAW filter to derive benefit from each system. Then comes the problem of packaging, to integrate an SAW filter and a BAW filter in the same system, with all the issues of space, weight and therefore associated cost and of optimizing the performance levels of each of the filters.
It has already been proposed to co-integrate a surface acoustic wave (SAW) filter and a bulk acoustic wave (BAW) filter in one and the same device, the SAW/BAW co-integration being done in the packaging by arranging side by side BAW and SAW filters obtained from two or more distinct fabrication lines.
With respect to the co-integration on one and the same substrate, it has been proposed, in the article by N. Lobo, D. C. Malocha: School of Electrical Engineering and Computer Science University of Central Florida, Orlando, Fla., 1051-0117/06, 2006 IEEE, Ultrasonics Symposium, to use common electrodes for the production of the SAW filter and for that of the BAW filter, the optimizations of said two filters not being able, by the same token, to be produced independently of one another.
There has also been proposed, in the U.S. Pat. No. 6,424,238 or in the patent application US 2007/0057772, a co-integration by approaches involving packaging two coupled but not very compact chips.
A different approach, described in the patent application US 2008/0284541, proposes the fabrication of a substrate for the production of a BAW. This patent application mentions the integration of other components (of LED, HEMT, and other such types). However, the same active layer is used for all of the components, which does not allow for the independent optimization of each of the filters, respectively SAW and BAW in the case in point.
It has also, finally, been proposed to co-integrate a BAW and a SAW on one and the same substrate with, for each of these filters, a distinct piezoelectric material according to the patent application US 2008/024245.